This invention relates to the art of induction heating and, more particularly, to a method and apparatus for inductively heating valve seat inserts of an engine component, such as an engine head.
The invention is particularly applicable for heating exhaust valve seat inserts in an aluminum cast engine head, and it will be described with reference thereto; however, it must be appreciated that the invention has broader applications and may be used for heating various types of valve seats in engine heads of various material and for heating similar types of work surfaces.
Internal combustion engines generally employ conically shaped valve seats which coact with reciprocating poppet valves for controlling flow of gases to and from the engine cylinders. During operation of the engine, the exhaust valves are subjected to extremely high temperatures and therefore experience a substantial amount of wear. To counteract this wear and increase the physical properties of the exhaust valve seat, the conical surface of the exhaust valve seat may be inductively heated during manufacture thereof. Subsequent to this inductive heating, the valve seat is quench hardened through conventional liquid or mass quenching. Induction heating of multiple valve seats in an engine component may be adapted to automatic processing of the engine head in a single operation such as by the use of a gang type induction heating apparatus wherein a like multiple of single turn inductors are individually positioned immediately adjacent to the conically shaped exhaust valve seats. Additionally, it is necessary to provide accurate and uniform magnetic coupling between the separate inductors and the conical surface of the valve seats.
U.S. Pat. No. Re. 29,046 discloses an apparatus for positioning circular inductors a common, preselected distance from conical surfaces of a body member, when such surfaces are to be inductively heated by the apparatus. The inductors are independently, reciprocably movable perpendicular to the body member or valve seats and biased in a direction toward the valve seats. A common frame from which the inductors are outwardly biased is moved into contact with the engine component and each inductor contacts the valve seat which it is to heat. Prior to moving the frame backwardly a distance generally corresponding to the desired magnetic coupling distance for the separate inductors, the inductors are locked to the frame. U.S. Pat. Nos. 3,761,669 and 3,777,096 assigned to the same assignee are likewise in point.
The above-identified U.S. Pat. No. Re. 29,046 is concerned primarily with induction heating of exhaust valve seats in a gray cast iron, engine head. As a direct result of efforts to increase gasoline mileage and the consequential reduction in weight of motor vehicles, aluminum is increasingly used in engine components. While the use of aluminum for the manufacture of the major components of the engine, engine block and head, provide substantial benefit in weight reduction of the total engine, the re-engineering of significant mechanical portions of the engine are necessitated by the use of aluminum in these major components. Aluminum heads of internal combustion engines, in particular, necessitate re-engineering the manufacturing methods used to provide valve seats therein for conventional overhead valve engine design. In this respect, the aluminum material cannot provide sufficient strength and hardness in order for efficiently wearing valve seat surfaces to be constructed therein.
In order to enable aluminum to be used in internal combustion engine heads, it has been determined that exhaust valve seat surfaces must be provided through the use of an insert constructed of hardenable metal material installed within the aluminum cast head through any suitable process, such as pressure applied force fit or integral casting within the head. Regardless of the method used to produce the aluminum head with metal inserts therein, machining of the inserts subsequent to casting or insertion is required to assure accurate location and concentricity of the valve seat surfaces. Since machining of the conically shaped valve seat surfaces is not possible once hardening of the material has occurred, the operations of machining and hardening must occur subsequent to installation of the inserts in the aluminum head. Machining of the conical surface of the insert does not present any unusual problems; however, hardening of the surface must occur without any deleterious effects to the pressure fit between the aluminum head and the metal insert.
The temperature customarily required to sufficiently harden an exhaust valve seat surface is approximately 1700.degree. F. Depending upon the particular alloy of aluminum used for casting the engine head, the melting point of the casting aluminum ranges from 1200.degree.-1400.degree. F. Inherent difficulties in hardening an exhaust valve seat insert previously provided within a cast aluminum engine head are readily apparent. Despite the fact that the required heat for hardening might be capable of being directed immediately onto the metal insert to be hardened, conduction of the heat through the insert to the cast aluminum occurs. If the heat to which the surrounding aluminum is subjected, during induction heating of the insert, results in the aluminum becoming expanded, a breakdown of the pressure fit between the metal insert and the cast aluminum head then is likely to occur. Thus, while hardened metal valve seat inserts in aluminum cast engine heads are necessary, and machining and subsequent hardening of the inserts in place in the head is desired, the temperature required to harden the insert heretofore has rendered the hardening impractical in the immediate environment of the cast aluminum.